There has been a long-standing and recently growing need for heat meters. Among the purposes are: for determining the efficiency of a heat exchanger, for evaluating the cost-effectiveness of solar heaters for domestic hot water or for residential heating by means of circulating hot air, for metering the amount of heat (rather than mere volume) in domestic hot water heated in a solar heat-absorber, and more recently for determining the amount of heat absorbed in a form of residential air conditioner served by circulating cold water.
With recent increases in the cost of oil, gas and electricity, the interest in solar heat has risen and, with it, the importance of related instrumentation. For example, it has been proposed to use solar heaters supplemented by electric or fossile-fueled heaters for providing domestic hot water. In such an application, an extremely important concern is to determine the cost-effectiveness of the solar heater in reducing the demand on the conventional hot-water heater which is used as a booster or as a stand-by heater. The investment in the solar heater is to be recouped over a period of time in the heat it delivers over the extended period. The monetary value of the solar heat can be computed in terms of the cost of electrical energy (for example) that would otherwise be needed to provide the heat. In that application, a heat meter is needed for accumulating the heat provided by a solar heat absorber, operable over several seasons of the year if not year-round. Evaluation and sale of solar heaters requires a dependable and accurate yet inexpensive heat meter.
In a related application, it was proposed to meter solar-heated hot water for domestic consumption not merely by measuring the hot water volume but by a heat meter that takes account of both the volume of the hot water and the temperature rise as a measure of energy consumption.
In recent years electromechanical and largely electronic heat meters have been proposed. Such instruments test the temperatures at the inlet and outlet of the heat exchanger, multiply the temperature difference by the metered fluid volume, and accumulate the result over a short or long period of time.